12 KiB
ambium
Analog brown-noise generator. Single +12 V supply, breadboard-friendly. Deep filtered rumble to a 4 Ω speaker via a cool-running Class-AB push-pull output.
Reference designators and model numbers are consistent across every diagram, the BOM, and the connection list — the connection list is the authoritative netlist.
Complete schematic
+12V
|
+-----------------+--------------------------+
| | |
R1 100k R2 10k U1 pin 8
| | |
o raw o----- VREF (6V) |
|\ | |
| E Q1 2N3904 R3 10k C2 100n
| B| C (NC) | |
| +-- GND GND GND
|
+-- C1 10u -- NOISE -- Rin1 1k --o SUM
+ - |
+---+---+
| |
Rf1 1M Cf1 8.2n
| |
+---+---+
|
+------------------------- AMP
| |
U1B pin 6 (-) U1A pin 3 (+)
VREF ------------------------------------------------ U1B pin 5 (+)
U1B pin 7 (OUT) -------------------------- AMP
U1A: LM358 output driver
OUT o--------------------- U1A pin 2 (-)
U1A pin 1 (OUT)
|
BASES
+----+----+
| |
B Q2 Q3 B
+12V ------- C PN2222 PN2907 C ------- GND
E E
| |
Re1 10R Re2 10R
| |
+----+----+
|
OUT
|
Cout 470u (+| |-) ---- SPKR ---- LS1 4R ---- GND
VREF bypass: VREF ---- Cref 10u (+| |-) ---- GND
Supply bulk: +12V ---- C3 47u (+| |-) ---- GND
U1 supply: pin 8 = +12V, pin 4 = GND
Parts
- U1 = LM358 (dual op-amp, DIP-8). U1B = amp + brown shaping. U1A = output driver.
- Q1 = 2N3904 (NPN). Avalanche noise source.
- Q2 = PN2222 (NPN). Push-pull top half (sources +).
- Q3 = PN2907 (PNP). Push-pull bottom half (sinks −).
Nets
+12V · GND · VREF (6 V mid-supply) · NOISE · AMP · SPKR
Signal chain
[noise] [amp + brown] [push-pull driver]
Q1 2N3904 → U1B LM358 −6dB/oct → U1A + Q2/Q3 → 4Ω speaker
NOISE AMP OUT (Class AB, cool)
Power · VREF divider (R2, R3, Cref)
+12V
│
[R2 10k]
│
●───────────────┬────────► VREF (6 V)
│ + Cref 10µF
[R3 10k] ──┤├──
│ −
│ │
GND GND
R2/R3 = 20 kΩ across 12 V, midpoint = 6 V. VREF = AC ground (single supply).
Stage 1 — White noise (R1, Q1, C1)
Reverse-biased B–E junction of Q1 into avalanche = broadband white noise. R1 limits current; C1 couples it out.
+12V
│
[R1 100k]
│
●──+┤├──────────► NOISE
│ C1 10µF (+ → emitter side; − → NOISE)
E
┌──┴──┐
GND─B─┤ Q1 │ 2N3904
└──┬──┘
│
C × no connect
NOISE = the AC-coupled output after C1. The ● node is raw noise on a DC pedestal — internal, don't label it.
Avalanche varies part-to-part. If quiet, swap the 2N3904. Needs full +12 V.
Stage 2 — Amplifier + brown shaping (U1B, Rin1, Rf1, Cf1)
Single inverting amp, low-frequency gain −Rf1/Rin1 = ×1000, centred on VREF. A feedback cap Cf1 across Rf1 makes it a leaky integrator: flat gain below the corner, then −6 dB/oct above it. That −6 dB/oct slope turns white noise into brown. One stage does gain and browning together — no separate integrator needed.
- corner fc = 1/(2π·Rf1·Cf1) ≈ 19 Hz with Rf1 1M, Cf1 8.2nF.
- below fc: flat ×1000. above fc: −6 dB/oct, reaching unity (×1) at ~20 kHz.
- so the whole audible band (20 Hz–20 kHz) rides the brown slope; peak gain ×1000 at the bottom, rolling off inaudible ultrasonics at the top for free.
Why not a flat ×1000 stage? LM358 GBW = 1 MHz → a flat ×1000 stage has only 1 MHz/1000 ≈ 1 kHz bandwidth, collapsing the band (measured ~27 mV pp at AMP). Here Cf1 makes the demanded gain fall with frequency, so the op-amp GBW is never the limit — the RC feedback does the shaping, not the op-amp rolloff.
LM358 DIP-8, notch at top, pin 1 top-left. Amp = unit B (pins 5/6/7).
┌─────∪────┐
OUT A 1 ─┤● ├ 8 V+ ──── +12V
IN− A 2 ─┤ ├ 7 OUT B ──●──────────────► AMP
VREF ─ IN+ A 3 ─┤ LM358 ├ 6 IN− B ──●
GND ─ V− 4 ─┤ ├ 5 IN+ B ──── VREF
└──────────┘
feedback (pin 7 ↔ pin 6): pin 7 ●──[Rf1 1M]────● pin 6 Rf1 ∥ Cf1
pin 7 ●──[Cf1 8.2nF]──● pin 6 brown slope, corner ~19 Hz
input: NOISE ─[Rin1 1k]──● pin 6
DC handling: C1 blocks DC into Rin1, so the op-amp's own offset sees unity gain — output sits at VREF (6 V), no railing.
Unit A (pins 1/2/3) = the push-pull output driver — see next section. Power/decoupling (whole chip, once): pin 8 → +12V, pin 4 → GND. 47µF bulk + 0.1µF at pin 8.
Output driver — Class-AB push-pull (U1A + Q2/Q3), 4Ω speaker
AMP is line level; the LM358 can't drive a speaker alone. Complementary emitter- followers inside the U1A loop: Q2 (NPN) pushes the positive half, Q3 (PNP) pulls the negative half. Both halves are actively driven, so there's no idle resistor and near-zero quiescent current → it runs cool. U1A drives both bases; global feedback from OUT hides crossover distortion (no bias diodes needed).
Driver = LM358 unit A (pins 1/2/3), same chip as the amp.
┌─────∪────┐
bases ─ OUT A 1 ─┤ ├ 8 V+ ──── +12V
OUT ─ IN− A 2 ─┤ LM358 ├ 7 (OUT B → AMP, Stage 2)
AMP ─ IN+ A 3 ─┤ ├ 6
V− 4 ─┤ ├ 5
└──────────┘
Push-pull output stage (totem pole):
+12V
│
C
┌───┴───┐
bases ──B──────┤ Q2 │ PN2222 (NPN — sources +half)
└───┬───┘
E
[Re1 10Ω]
│
●──────────► OUT ──+┤├──► SPKR ─ GND
│ Cout 470µF 4Ω
[Re2 10Ω] (+ → OUT)
E
┌───┴───┐
bases ──B──────┤ Q3 │ PN2907 (PNP — sinks −half)
└───┬───┘
C
│
GND
Connections:
input: AMP ─────────────● pin 3 (IN+ A)
drive: pin 1 (OUT A) ───● bases → Q2.base + Q3.base
feedback: OUT ─────────────● pin 2 (IN− A)
Q2: C → +12V B → bases E → Re1 → OUT
Q3: C → GND B → bases E → Re2 → OUT
OUT node = Re1 ● Re2 ● pin 2 (fb) ● Cout.+ ; Cout.− → speaker → GND
- Idle ≈ 0 → cool, no heatsink. Re1/Re2 = 10 Ω ¼ W (4.7–10 Ω ok; carry only AC — not hot).
- OUT sits at ~6 V DC (follows AMP); Cout 470 µF blocks it, speaker sees AC only.
TO-92 pinout (flat face toward you, leads down): PN2222 = E-B-C, PN2907 = E-B-C (confirm against your batch's datasheet).
- PN2222/PN2907 cap ~600 mA → ~1 W into 4 Ω before current-limiting. Plenty for the rumble, and cool. Want more power → TIP120+TIP125 pair, or an amp chip.
- Crossover buzz at low level? Split the bases and add 2× 1N4148 between them for bias. Usually unneeded — feedback handles it for a noise source.
Bill of materials
| Ref | Value | Part | Function |
|---|---|---|---|
| U1 | — | LM358 (DIP-8) | U1B amp+brown, U1A output driver |
| Q1 | — | 2N3904 | avalanche noise source |
| Q2 | — | PN2222 (NPN) | push-pull top (source) |
| Q3 | — | PN2907 (PNP) | push-pull bottom (sink) |
| R1 | 100k | resistor | avalanche current limit |
| R2, R3 | 10k | resistor | VREF divider |
| Rin1 | 1k | resistor | amp input |
| Rf1 | 1M | resistor | amp feedback (low-freq gain ×1000) |
| Re1, Re2 | 10Ω ¼W | resistor | push-pull emitter degeneration (4.7–10Ω ok) |
| C1 | 10µF | electrolytic | noise coupling |
| Cref | 10µF | electrolytic | VREF bypass |
| Cf1 | 8.2nF | film / ceramic | feedback / brown shaping (−6dB/oct, corner ~19 Hz) |
| Cout | 470µF ≥16V | electrolytic | speaker coupling (big, for lows) |
| — | 0.1µF | ceramic | LM358 decoupling |
| — | 47µF | electrolytic | +12V bulk |
Chip count: 1× LM358 + 1× 2N3904 + PN2222/PN2907 pair. No heatsink needed (Class AB).
Connection list (authoritative netlist)
+12V : R1.1, R2.1, U1.8, Q2.collector, 47µF+
GND : R3.2, Q1.base, Cref.−, U1.4, Q3.collector, 47µF−, 0.1µF
VREF : R2.2, R3.1, Cref.+, U1B.IN+(5)
(src) : R1.2, Q1.emitter, C1.+ (internal raw-noise node)
NOISE : C1.−, Rin1.1 (after C1)
AMP : U1B.OUT(7), Rf1, Cf1, U1A.IN+(3) (Rin1.2, Rf1, Cf1, U1B.IN−(6) = summing)
bases : U1A.OUT(1), Q2.base, Q3.base (driver output → both bases)
OUT : Q2.emitter→Re1 ; Q3.emitter→Re2 ; Re1.2 = Re2.2 = OUT node
OUT → U1A.IN−(2) (feedback) ; OUT → Cout.+ ; Cout.− → speaker → GND
Q1 : collector = no-connect
Build & test order
- VREF — confirm ~6.0 V.
- Noise (R1, Q1) — scope Q1 node → hiss (~mV). None? swap 2N3904 or raise supply to 12–15 V.
- Amp (U1B) — scope AMP → deep rumble (~volts), darker than the raw Q1 hiss (that's the brown slope).
- Driver (Q2/Q3) — wire push-pull + feedback. Speaker rumbles. Transistors stay cool (near-zero idle). OUT DC ~6 V.
Tuning
| Want | Change |
|---|---|
| Louder | ↑ Rf1 (raises low-freq gain; watch clipping) |
| Darker / deeper | ↑ Cf1 (lowers corner, more brown) |
| Brighter (toward white) | ↓ Cf1 (raises corner) |
| Kill crossover buzz | add 2× 1N4148 base-bias diodes |
| More power | TIP120+TIP125 pair, or amp chip (PAM8403 / TDA2822) |